[jeff_banning]

The blackwater flush anti-siphon valve under the sinks in all brands RV's have always been an issue. Mine had a cracked line connection, brand new one would not flow water at all, I pulled guts (ball and spring) and now it flows. Would be tough to back flow when tank is empty but I get why they are on there. Many bypass them.

Several have methods for attempting to clean/save sensors. I got my sensors on a camper to read correctly after filling tank with water (garden hose down stool) but first added 2 gallons of bleach and a big bottle of fabric softener and letting it set for couple of days. Others say use calgon, dawn dish soap, and flush. Some have said a box of septic tank yeast, fill with water an let it eat at stuff and flush. Many methods listed around attempts to clean the tank and get sensors working.